DIGGING DEEPER: SINGLE-SKIN METAL PANEL PERFORMANCE

Performance and Sustainability at the Exterior

To be sustainable, buildings must be able to withstand the trials of climate, seasonal change, and hazardous weather events. Preventing moisture intrusion is a critical requirement for a building envelope’s exterior layers. The envelope must also successfully address the wear and tear from air and thermal conditions over time. When dealing with defense mechanisms against water leakage, architects must pay heed to Murphy’s Law: “If anything can go wrong, it will.” Nothing is more aggravating to a client, short of a total building collapse or the burning of a structure, than the occurrence of a leak. Proven exterior wall system approaches that successfully control moisture, air, and thermal conductance fall into two categories: conventional wall systems and rainscreen wall systems.

Conventional wall systems can further be divided into single-line barrier walls and secondary gutter backups. Rainscreen wall systems are designed either as drained/back-ventilated systems or as pressure-equalized/compartmentalized systems.

Under conventional strategies, single-line barrier walls, also referred to as barrier walls and face-sealed walls, act as an all-in-one layer to keep out rain, as well as retain conditioned interior air and resist the forces of wind and air pressure. This type of wall can additionally act as a vapor barrier. Examples of single-line barrier walls include EIFS, stucco, and some insulated metal panel systems.

The moisture control of these types of systems is completely reliant on the performance of joint sealants, which may be butyl, gaskets, or silicone sealants. Frequently, these joints use a wet joint, which is a surface-sealed system using a wet joint that is labeled to act as a rainscreen and, in reality, acts as a barrier wall. Consequently, since single-line barrier systems rely on seal integrity at joints, so the workmanship at these joints is critical for long-term performance. Additionally, wet joint systems sealant materials remain tacky, making them susceptible to airborne dirt collection at the joints. When it rains, this accumulated dirt and debris streaks and can potentially stain the exterior finish.

Secondary gutter backup is the other category of a conventional wall system approach. To achieve this approach, a secondary drainage system is added behind the primary system and made integral with the single-line barrier skin. Examples include aluminum and glass curtain-wall systems. This system is highly dependent on the integrity of internal seals and workmanship of installation.

Rainscreen wall systems take a different approach to the exterior wall and building envelope.

The Rainscreen Principle was first defined in the 1988 book Rainscreen Cladding: A Guide to Design Principles and Practices by JM Anderson and JR Gill. The Rainscreen Principle is based on two distinct barriers or leaves: the outer leaf and inner leaf. The outer leaf sheds and controls water but may not necessarily eliminate the majority of the rainwater. The inner leaf has multiple functions, including acting as the final moisture barrier, the air/vapor barrier, providing insulation, and serving as the building’s structural wall.

Two distinct approaches to weather protection in rainscreens have emerged. The drained and back-ventilated rainscreen strategy involves draining off most of the rainwater at the outermost surface of the wall and providing for cavity drainage and evaporation for the remainder.

Elements of the drained and back-ventilated rainscreen include:

• Cladding (sheets, panels, or planks) is fixed to support channels or rails as the outermost exposed leaf or barrier.
• Joints in the outer leaf are open but designed to minimize (not prevent) water penetration by kinetic force.
• Water is permitted to run down the interior face of the outer leaf cladding.
• A cavity (or minimum allowable width of air space) between the outer leaf and inner leaf is necessary to facilitate positive back-ventilation.
• An air/water barrier at the rear of the cavity (outer face of inner leaf) is required.
• The inner leaf must be completely flashed.

Since drained and back-ventilated rainscreen systems are not pressure equalized, the exterior leaf must be designed to withstand 100 percent of the building’s wind load. Examples of outer leaf cladding include metal, stone, wood, and terra cotta.

Single-Skin Metal Panels as a Rainscreen

As opposed to the drained and back-ventilated rainscreen, the goal of the pressure-equalized rainscreen is to eliminate penetration through the rainscreen not by tightly sealing joints but rather leaving some or all of them open to the passage of air but not of water.

“Joints between panels are the weak element in the defense against leaks. Systems that depend on the barrier wall approach using sealants are the least sophisticated. Those that utilize the rain screen principle with pressure equalization stand a better chance of performing faultlessly over a long period of time,” writes Fred Nashed, AIA, author, Exterior Wall Design. The pressure-equalized rainscreen may also be referred to as pressure-equalized/compartmentalized (PE/C) rainscreen.

Building envelope elements under the pressure-equalized rainscreen approach include:

• Outer leaf cladding system (or the rainscreen);
• Vertical drainage channel;
• Penetration flashing;
• Ventilation cavity/compartment;
• Moisture barrier (commercial air/vapor barrier);
• Approved moisture-barrier compatible flashing at all penetrations;
• Inner leaf or face of building structural wall;
• Optional moisture resistant insulation;
• Ventilation path for pressure equalization and drainage;
• Horizontal air dam used to create compartmentalization; and
• Building structural wall.

The pressure-equalized rainscreen offers certain advantages for building life cycle and maintenance. Pressure-equalized rainscreen systems use true dry joints, which do not rely on sealants or gaskets. No sealants or gaskets means no maintenance legacy for the building owner, and no possibility for dirt accumulation or surface staining. Additionally, the outer leaf panels, when single-skin metal panels are selected, are not laminated or a composite, meaning they will never delaminate.

When composite panels or wet joints are part of the rainscreen design, design professionals must watch for failures, including delamination, staining due to the effects of weather on joint sealants and gaskets, and inefficient installation practices. Composite systems may also limit color, texture, and design options.

While both rainscreen approaches can work successfully, care must be taken that the two are not confused, since the adoption of hybrid versions can result in unsatisfactory performance.